CHITONID^E. 245 



more subrotund. The auricles receiving blood from the bran- 

 chial veins communicate in the usual lateral manner with the 

 greater division of the heart, and at this point present their 

 largest calibre ; they then form an attenuated arcuation on 

 each side the constricted portion, and effect a second contact 

 at the sides of the lesser inflation, which may be regarded as 

 an aortic ventricle receiving the blood by a special auricular 

 apparatus. From the anterior axis of the major part of the 

 heart, a long and large aorta or arterial vein ascends medially 

 to the front, distributing by diverging arteries the aerated 

 fluid to the greater portion of the body, whilst the minor and 

 posterior ventricle, after receiving from the arcuated ducts of 

 the auricles its quota of blood, serves the remaining area ; but 

 I could not detect a descending arterial vein, which however 

 must exist to convey the blood ; unless one of the arteries of 

 the ascending aorta doubles back and supplies the part. But 

 this conjecture is scarcely tenable, as we must then suppose, 

 that the blood conveyed by the arcuation of the auricles to the 

 lower inflation is thrown back again on them and the larger 

 ventricle, thus producing a useless periodic action and counter- 

 action. 



We will endeavour to explain the probable causes and effects 

 of this unusual construction. It has been shown that the 

 principal ventricle of the heart is the propelling power of the 

 blood to the major part of the body, and the smaller one by 

 its separate auricular contact supplies the remainder ; we have 

 also observed on the connection of both parts of the heart by 

 the strangulated and without doubt valvular intervening por- 

 tion, so that no blood can pass between the two ; and it is 

 clear that none is intended to pass, as the auricles by distinct 

 ducts supply each with its proportion of the vital fluid : con- 

 sequently the mitral valves of the ventricles are closed at their 

 axes abutting on the strangulation at each simultaneous dila- 

 tation, and thus this constriction acts as a point d'appui and 

 of departure, and enables them to exercise a full power of 

 propulsion. 



We shall now perceive the reason for these separate infla- 

 tions. If one only had existed, with an ascending and de- 



